The present invention relates to a display device comprising a cathode ray index tube and also to a deflection system for use in a cathode ray index tube.
Many different display devices comprising cathode ray tubes are currently known. The most common type of cathode ray tube comprises a shadow mask, arranged on the inner side of the screen above a phosphor layer. The shadow mask has the function of shadowing two of the three phosphor areas when the third is being illuminated by an electron beam. Thus, when the “red” electron beam is activated, the shadow mask shadows the green and blue phosphor areas, etc. The shadow mask has several drawbacks, for example, it is costly and absorbs roughly 80% of the electrons emitted from the gun.
Consequently, cathode ray tubes have also been developed without shadow masks, sometimes referred to as beam indexing CRTs or intelligent tracking CRTs.
In such tubes, known from e.g. U.S. Pat. No. 3,914,641, the different phosphor areas are arranged in groups of strips, normally with a horizontal extension across the screen. The electron beams are deflected to land on the correct strips, commonly by means of a system of electrodes located adjacent and in between the phosphor strips. According to the most commonly used arrangement, two electrodes with elongate finger portions are arranged in such a way that each phosphor strip is located between finger portions belonging to different electrodes. Each electrode is arranged to detect a signal resulting from electron beams landing on the electrode, and the signals from the two electrodes are compared (e.g. subtracted or divided). The relationship between these signals is used to control the beam deflection unit in a feedback position control system.
Basically, there are two different categories of intelligent tracking CRTs, namely:
1) single beam systems, with only one electron gun, alternately illuminating phosphor strips of different colours, and
2) multi-beam systems, where several electron guns are employed, each illuminating one of the phosphor strip groups. The multi-beam systems of course have the advantage of writing red, blue and green information in one sweep.
The present invention is related to the latter category.
U.S. Pat. No. 3,914,641 discloses a multi-beam system having an electron gun assembly is arranged in the neck of a cathode ray tube. The electron gun assembly produces a multiple of, for example three electron beams, each intended to illuminate one of the above-described phosphor strip groups arranged on the inner side of the cathode ray tube screen. In order for each beam to hit the correct phosphor strip, the beams are vertically separated from each other as they hit the screen, i.e. provided with a so-called convergence error, in this case a vertical convergence error, because said strips are horizontally arranged. In order to deflect the beams to the correct spot on the screen, a deflection unit is arranged around the neck of the cathode ray tube, between the electron gun assembly and the screen. To provide said vertical convergence error, a quadrupole (such as a 90-degree quadrupole) is arranged around the neck of the cathode ray tube, close to the electron gun assembly.
A problem with this kind of display is, however, that it is difficult to maintain a constant vertical separation between the electron beams as they hit the screen, after they have been deflected. The reason for this is that the sensitivity of the quadrupole changes as a function of the deflection. This has usually the effect of a degraded picture quality around the edges of the display screen. One way to solve this problem is to operate the quadrupole with a dynamic signal taking the angle of deflection into consideration. This is, however, a somewhat complicated solution, especially since the amplitude of the dynamic signal tends to get large, and consequently there is a need for a simpler and more inexpensive way of attending to this problem.
Thus, it is an object of the present invention to provide a display device that overcomes the above-described problems with the prior-art device.
This and other objects are achieved by a display device comprising a cathode ray index tube having a neck portion and a screen portion, the display device comprising an electron gun assembly for generating a plurality of electron beams, positioned in the neck portion of the cathode ray index tube, a deflection unit, placed around the neck portion of the cathode ray index tube, for deflecting said plurality of electron beams, and a convergence unit for converging said plurality of electron beams, said deflection unit being placed at an axial distance from said electron gun assembly, closer to said screen portion, wherein said convergence unit is placed in close proximity to the deflection unit. Placing the convergence unit close to the deflection unit has the advantage that the uniformity (or effect as a function of the location on the screen) of the vertical beam variations varies with the axial position where the effect is created. The effect is virtually constant near the deflection centre of the deflection unit. Consequently, by placing the deflection unit and the convergence unit close together, a virtually constant vertical convergence error may be achieved. Furthermore, by placing the convergence unit close to the centre of deflection of the deflection field created by the deflection unit, one evades the above-mentioned problem that very large currents are needed, for CRTs with large deflection angles, in order to provide for a constant convergence error on the outskirts of the screen.
In accordance with a preferred embodiment, said convergence unit is placed at essentially the same axial distance from the electron gun assembly as the deflection unit. This embodiment further enhances the system. Moreover, the convergence unit may suitably comprise one or more windings wound around a yoke core in said deflection unit. By winding the windings of the convergence unit around the yoke core of the deflection unit, a very good centring of the deflection and convergence units is achieved. Moreover, a compact construction is obtained. Moreover, said convergence unit may comprise two or more windings wound around separated core parts of a split yoke in said deflection unit. By using two or more separated windings, which may be controlled separately, a very good uniformity can be obtained, resulting in a constant or virtually constant convergence error.
The above-mentioned and other objects are also achieved by a deflection system for use in cathode ray index tubes having a neck portion and a screen portion, the deflection system comprising a deflection unit, intended to be placed around the neck portion of the cathode ray index tube, for deflecting electron beams generated by an electron gun assembly in said index tube, and a convergence unit for converging said plurality of electron beams, wherein said convergence unit is placed in close proximity to the deflection unit. Placing the convergence unit close to the deflection unit has the advantage that the uniformity (or effect as a function of the location on the screen) of the vertical beam variations varies with the axial position where the effect is created. The effect is virtually constant near the deflection centre of the deflection unit. Consequently, by placing the deflection unit and the convergence unit close together, a virtually constant vertical convergence error may be achieved when said unit is used in a cathode ray index tube.
The convergence unit and the deflection unit are preferably positioned in such a way that their respective centre points essentially coincide. In doing so, the above-described effect of the correlation between the sensitivity of the quadrupole and the deflection is minimised. In accordance with a first embodiment of the invention, the convergence unit may comprise one or more windings wound around a yoke core in said deflection unit. By winding the windings of the convergence unit around the yoke core of the deflection unit, a very good centring of the deflection and convergence units is achieved. Moreover, a compact construction is obtained. In accordance with a second embodiment of the invention, said convergence unit may comprise two or more windings wound around separated core parts of a split yoke in said deflection unit. By using two or more separated windings, which may be controlled separately, a very good uniformity can be obtained, resulting in a constant or virtually constant convergence error.